Heat exchangers such as automotive condensers and evaporators are typically brazed aluminum parts, in which constituent components covered with a braze layer are run through a braze furnace. The furnace melts the braze layer which, when liquid, is drawn by capillary action into various narrow component interfaces, later cooling and hardening to create a solid part. This process works well only if the inside of the braze furnace and the internal atmosphere surrounding the part can be kept substantially oxygen free, typically to within only a few parts per million. Clearly, some means must be found to prevent the leakage of oxygen rich, outside air into the furnace. Just as clearly, the inside of the furnace must, at least at some point, be open to the outside for the entry of, and removal of, workpieces.
The simplest approach to outside air exclusion is not the most productive. In so called batch processing, a discrete number of workpieces are placed in the furnace, which is then sealed off and evacuated. When finished, the doors are opened and the parts removed. Another batch processing system does not use a vacuum, but instead injects an inert atmosphere of nitrogen to purge the outside air after the doors are closed. Batch processing of whatever kind is clearly inferior, in terms of parts per hour, to a system in which parts can be run continually through the furnace, with no stopping or starting to open or close doors. A flow through process, on the other had, presents the dilemma that the furnace must be continually open to the outside, providing a constant leak path for oxygen into the furnace.
Known flow through furnaces, such as that shown in co assigned U.S. Pat. No. 5,172,847, exclude most outside air by injecting inert gas, typically nitrogen, into the furnace interior, creating an always positive pressure balance that sacrificially leaks through the vestibule openings continually. In an attempt to at least reduce the rate of sacrificial leakage and the rate of expensive nitrogen usage, barrier "curtains," depending strips of flexible stainless steel, are hung at the open vestibule doors. These continually drag over the parts flowing past on the conveyer, impeding their progress in addition to impeding air exchange. The curtains inevitably wear with use, and are expensive to replace. In addition, they limit the number of designs possible for the parts themselves, obviating the use of protruding pipes or the like that might catch and grab on the curtain.